Radar System Engineering

550 THE RECEIVING SYSTEM-INDICATORS [SEC.1321
phosphorescent light, sothatcontrast isreduced byreflectedhght. The
necessary thickness of the screen tends to blur the edges of the signals
and thus reduce contrast gradient, as do also limitations in the sharpness
of focus of the electron beam. Many screens also have a certain amount
of graininess which contributes to the reduction of contrast gradient.
Contrast between signals of different intensities is especially important
in overland flying, where it is essential to be able to distinguish land
from water and at the same time to have good contrast between weak
echoes from ordinary terrain and the much stronger ones from built-up
areas. Unfortunately the range of echo intensities involved is so great
that, if the receiver has gain high enough to render the terrain visible,
the signals from built-up areas must be drastically limited to avoid
“ blooming” of the cathode-ray tube, and detail is destroyed. If the
gain is low enough to preserve detail, the land background is not visible.
This difficulty is made worse by the nonlinear response of the cathoderay
tube. As was previously mentioned, the beam current in a magnetic
tube is proportional to the cube of the grid-voltage swing measured from
cutoff, which is the operating point in the absence of signal. This
means that a signal of, for example, half the voltage of a limited signal
will, in principle, give only one-eighth as much light intensity. In actual
fact the difference is not so great as this, because of the tendency of the
screen to saturate on strong signals. This nonlinearity reduces considerably
the difference in voltage between those signals that are just
strong enough to produce a visible result and the level at, which strong
signals must be limited. It is possible to introduce a nonlinear element
in the video amplifier to compensate for the tube characteristic, and this
has been done with some success. No really satisfactory circuit of this
sort has been devised, no doubt because the need for it was fully appreciated
only recently. Even with a linear CRT grid characteristic,
the problem of narrow dynamic range would be a serious one. General
methods of attacking it will now be described.
13.21. Special Receiving Techniques for Air-to-land Observation.—
Some improvement in the ability to distinguish land from water and
at the same time to see detail in built-up areas can be effected by using
the anticlutter techniques described in Sec. 12.8. Fast time constants,
instantaneous automatic gain control, and other such circuits tend to
suppress saturation on long blocks of signals and to make full use of
changes in the signal intensity. Unfortunately the use of such measures
results in an unnatural appearance of the display which makes interpretation
exceedingly difficult. Shorelines, for example, stand out
very strongly, as do changes in the terrain. Similarly a solidly built-up
area appears chiefly in outline, with other strong signals at points of
changing signal intensity. Consequently, although they are extremely